Laboratories around the world, many represented here, have now elucidated the broad outlines of the dendritic cell system. Dendritic cells occupy three compartments: a) nonlymphoid tissues such as the Langerhans cells of the skin and the interstitial dendritic cells of lung and heart, b) the circulation especially the veiled cells of the afferent lymph, and c) lymphoid organs primarily the interdigitating cells of the T cell areas and thymic medulla. These pools of dendritic cells are interconnected, since nonlymphoid cells can pick up antigens and migrate into the blood or lymph and to lymphoid organs.